As described in the present assignee's previous work, lenticular lenses, or lenticules, are typically cylindrical bodies having longitudinal axes and arranged in a parallel-axis array on a lenticulated face of a lenticular sheet. The face opposite the lenticulated face typically is substantially planar. Seen in cross-section, each lenticule has a vertex distal from the planar face, and adjacent lenticules intersect to define valleys proximal to the planar face. A lenticular height is defined between a first plane tangent to the lenticule vertices and a second plane tangent to the lenticule valleys. A lenticular pitch is defined between axes of adjacent lenticules along a raster axis perpendicular to the axes of the parallel lenticules.
The lenticular sheet typically is formed with a thickness of the sheet being substantially equal to a focal length of the cylindrical lenses or lenticules. The graphic resolution along the raster axis is then limited to the lenticular pitch. In the finished lenticular product, the planar face commonly carries a specially prepared and registered printed image. The image is most usually printed directly to the planar face of the sheet, but may also be formed on a separate substrate, and then aligned and adhered to the planar face.
A tradeoff between quality of focus and viewing angle is well known in the lenticular art. The influence of refractive index is also well understood. Lenticular sheets are often described according to the lenticular pitch in lenses per inch. A 150 lens-per-inch (LPI) array is colloquially understood to be a fine pitch. 75 LPI lens is considered an industry standard. A 40 LPI lens has a relatively coarse pitch, generally used for applications in which the lenticular item is to be viewed at greater than arm's length. The majority of commercial applications are currently served by lenticular sheets having proportions between 1.2 times as thick as the lenticular pitch, to twice as thick as the lenticular pitch. A single lenticule of a 75 LPI lenticular sheet is about 339 microns (13 mils) wide from valley to valley. In its most common present commercial form, a 75 LPI lenticular sheet will have a refractive index of around 1.57 and a thickness of around 469 microns (18 mils), therefore being about 1.4 times as thick as the nominal lens width.
It may be understood that some applications have called for more extreme proportions, as when a thin, conformable lenticular label is required, in which case the proportion may be 1:1 or less. Conversely, superior optical resolving power is often sought after in autostereoscopic “3-D” display, and in this case the ratio of thickness to lens width may be 3:1 or greater. The preceding values descriptions are intended to characterize underlying principles, and identify the most readily available commercial materials in the current trade, and should not in any way be taken to limit the scope of the invention.
Lenticular sheets may be formed by any suitable method. For example, U.S. Pat. Nos. 5,330,799 and 5,554,532 to Sandor et al. describe a lenticular system in which lenses are formed upon a flat carrier sheet in a forming process which is commonly known as “cast film” lenticular. Sandor et al. describe lenses formed in local areas by forming and curing fluid material over the desired image areas.
Within the field of lenticular printing, there are a number of different processes and techniques for performing a printing operation depending upon a number of different factors including the nature of the final product and the intended application. Some of the more common printing processes include offset printing, lithographic printing, etc.
It will be appreciated that there are other types of printing techniques, that are used in different applications beyond the field of lenticular products. For example, one type of printing technique is a printing process which uses an anilox roll. An anilox roll is essentially a specialized gravure roll which has been uniformly engraved with a regular recessed pattern of cavities known as cells. The pattern increases the amount of ink that can be borne by a nonabsorbent roll face. The uniform engraved cells inherently meter the amount of ink taken up by the roll. Retention of the ink within the cells prevents sagging and dripping during or between printing cycles.
Anilox cells are incuse and arrayed in regular patterns of predetermined frequency. They typically have uniform depth and shape as well. The cylinder face may be engraved by mechanical tooling or by laser ablation.
The amount of ink delivered by the anilox roll is determined by the total volume of its cells. Anilox rolls can apply a relatively heavy and relatively uniform ink layer in a single operation. Because the anilox pattern covers the entire roll, they are commonly limited to blanket-coating operations.
Another type of printing process is flexographic printing (flexography). Flexography is a rotary printing method that uses plates of resilient material to print an image that can be received on diverse types of materials. In this process, ink is first transferred through a metered supply roll to the raised relief areas of the printing plate, and thereafter from the raised relief areas to the printed surface.
Because the resilient plate can conform to the receiving material, flexographic printing has long been used in the printing of corrugated cardboard and other irregular surfaces. The relief design in the flexographic plate is commonly generated in a photopolymeric plate material. Radiation exposure through a mask results in selective photopolymerization that ultimately results in a resilient, raised pattern.
The developed flexographic plates are typically then mounted on a printing cylinder. An ink reservoir supplies ink directly to an intermediate ink supply roll. A doctor blade can be electively employed to limit ink film thickness upon the supply roll. Ink is transferred first from the reservoir to the ink supply roll, and then from the ink supply roll to raised areas of the flexographic roll.
The mounted flexographic printing plate is then brought against the receiving medium with sufficient pressure to allow contact between the raised design on the plate and the receiving print medium. The print medium is typically fed between the flexographic roll and an unpatterned backing roll. Printing occurs as the pressure transfers ink from the flexographic plate to the print medium.
Photopolymeric flexographic printing plates can be made of a variety of radiation-sensitive polymer materials. Photosensitive polymer resin plates are available in the trade under the trademarked name CYREL. Various types of CYREL plates are offered by of E.I. DuPont de Nemours and Co., Inc. Development of these photopolymers typically occurs under ultraviolet radiation, but plates may be cured by any suitable source of actinic radiation.